Wireless communications systems (e.g., satellite networks) provide a pervasive and reliable infrastructure to distribute voice, data, and video signals for global exchange and broadcast of information. These communications systems have emerged as a viable option to terrestrial communications systems. Unlike terrestrial networks, such systems are more susceptible to service disruptions stemming from changing channel conditions, such as fading because of weather disturbances. Additionally, such systems cannot readily increase capacity, and thus, rely on various techniques to enhance spectral efficiency. Further, because of the diversity of communications needs of subscribers, subscribers demand that these wireless communications systems provide assurances with respect to availability, reliability, and throughput in a cost-effective manner. If a service provider supplies a uniform level of service, then more demanding subscribers would not be satisfied for a “standard” service guarantee, even though such subscribers are willing to pay a greater premium for better service; under this scenario, the service provider is foregoing revenue. However, if the uniform level of service is greater than what the less demanding subscribers seek, then these subscribers are essentially “overpaying” for the level of service, as they may be willing to pay less for fewer guarantees regarding network performance.
Wireless communications systems often employ multiple terminals to share a resource of spectrum; such a system is referred to as a multiple access system. Many methods of providing multiple access are deployed in existing communications systems; such as a Frequency Division Multiple Access (FDMA) system, and a Time Division Multiple Access (TDMA). With FDMA, a band of spectrum is supplied to each terminal based on frequency assignments, thereby allowing the terminals to share the spectrum. The TDMA scheme permits sharing the spectrum based on time; that is, when a terminal allowed to transmit.
Many different methods have been employed to allow terminals to perform multiple access in TDMA and FDMA/TDMA systems. These multiple access policies are typically evaluated in terms of their efficiency and their latency characteristics. Often, as the efficiency increases, the latency characteristics degrade. Efficiency, in this context, means the total number of usable bits carried divided by the total of each channel's information rate. An important example of a latency characteristic is the percentage of transmissions which are successfully delivered in less than a specific threshold latency, for example, 1200 msec
One multiple access scheme is referred to as Slotted ALOHA, which is conventionally used in a satellite system. With slotted ALOHA, the channels in a FDMA/TDMA system or the single channel in a TDMA system is divided into fixed size timeslots, referred to as ALOHA slots. When a terminal in the satellite system has data to transmit, the terminal selects an ALOHA slot at random and transmits the data within the ALOHA slot. The transmission within a slot is referred to as a burst. Frequently, when the system is lightly loaded, the terminal selects a slot that is not used by another terminal in which case the burst is considered successful. The term “collision” refers to the event in which two or more terminals attempt to transmit in the same selected slot. These transmissions interfere with each other, and in most cases none of the bursts transmitted in the slot are successfully delivered. When a terminal detects that its burst has failed, which typically occurs because of a collision, the terminal randomly picks another slot and retransmits. Provided the system is not overloaded, the terminal should within a small number of tries have its transmission succeed.
Based on the foregoing, there is a need for a radio communications system that provides a quality-of-service (QoS) mechanism in a shared medium using a multiple access scheme. There is also a need to enhance spectral efficiency. There is a further need to provide cost-effective network services to subscribers that are tailored to the subscribers' needs. Therefore, an approach for supporting differentiated network services is highly desirable.